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Best method to find number of times a 2D pattern occurs in a matrix ?
By Fear_Is_An_Illusion, history, 9 years ago, In English

pattern is fixed

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9 years ago, # |
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Can you give an example ?

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    9 years ago, # ^ |
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    matrix

    a b o z q
p z q 1 3
p 1 3 z q
a b c d e

    pattern

    z q 
1 3

    verdict : yes, 2 times

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      9 years ago, # ^ |
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      is overlapping allowed ?

      For example :

      a a a a a

      a a a a a

      a a a a a

      pattern

      a a

      a a

      Should the answer be 2 or 8?

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        9 years ago, # ^ |
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        lets not allow it :P

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          9 years ago, # ^ |
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          maybe slow solution. nP, mP — sizes of pattern. let's say position(i, j) is good if patern occurs in it. between every 2 good positions (i, j) and (i2, j2) add edge if rectangles (i, j, i + nP — 1, j + mP — 1) and (i2, j2, i2 + nP — 1, j2 + mP — 1) are intersect. Now problem is: given graph, find max subset of verticles, such that there r no 2 verticles u, v in this subset such that u, v are connected by edge. It's well known problem.

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            9 years ago, # ^ |
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            I think it's even slower than the brute force, since maximal independent set is NP-Hard... or did i get something wrong?

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9 years ago, # |
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Use hashes, just like when you find a substring in a string. Can't write more details now.

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    9 years ago, # ^ |
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    Convert each row of your pattern to a single hash value. If your patter has row length = n, then convert every substring of length n in each row of your matrix to a single hash value.

    Now the problem is to find a substring in a string along the columns for which you can use KMP or hashes again.

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9 years ago, # |
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Lets assume the array is of size N1*M1 and the pattern is of size N2*M2 1<=N2<=N1<=M2<=M1<=1000000 && N1*M1<=1000000 && N2*M2<=1000000.

We are not consider overlaps.

You need a visited array to make sure that you don't count overlaps and avoid checking a row and column more than once.

for(int i=0;i<=(n1-n2);i++){
    for(j=0;j<=(m1-m2);j++){
        if(!vis[i][j]){
           answer+=checkPattern(i,j); 
        }
    }
}

In check pattern , we'll return 0 if the pattern starting from i,j does not match. Else if it matches, we'll mark all the pattern matching cells as visited and return 1.

Update : Please let me know what is wrong guys.

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    9 years ago, # ^ |
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    Your obvious solution has a big complexity, which's definitely not what he needed.

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9 years ago, # |
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Take a look at KMP algorithm.

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